The loudspeaker enclosure, sometimes called a baffle, plays a critical acoustical role in sound reproduction. The quality and characteristics of the sound actually heard by the listener is influenced to a large extent by the speaker enclosure. Without the loudspeaker enclosure almost all of the low frequency sounds would be lost to the listener and even the best speaker would sound thin and reedy. The reason for this is that both the front and the back surfaces of a conventional cone-shaped speaker emit sound waves as the cone vibrates back and forth. Of course, the cone vibrates in response to the electrical signal received, for example, by the coil in a conventional electro-mechanical speaker. The electrical signal is generated by conventional audio equipment forming part of the total sound reproduction system. When the cone is driven in the forward direction, it compresses the air in front of the cone and causes a partial vacuum behind the cone. Similarly, when the cone vibrates backwards, the reverse occurs. Thus, the sound waves emitted from the front and back surfaces of the cone are always emitted out of phase because the air is compressed on one side and rarefied on the other. If the front wave is permitted to meet the back wave while they are still out of phase, the two waveforms acoustically cancel and no sound is produced. The phenomenon of acoustic cancellation can occur at any frequency, but it is most noticeable at low frequencies.
A major purpose of an enclosure, therefore, is to prevent the acoustic cancellation of sound waves from the front and back of the speaker. There are several different types of speaker enclosures which attempt to eliminate acoustic cancellation. Perhaps the simplest speaker enclosure is the sealed box type. In theory, since the enclosure is sealed, the sound wave which is radiated from the back surface of the speaker cone is totally contained within the enclosure. However, in a conventional rectangular speaker enclosure with the speaker mounted in a plane parallel to the front surface of the enclosure, the sound waves from the back of the cone contact the parallel interior back wall surface of the enclosure and reflect back towards the speaker cone. A portion of the reflected sound waves radiates through the cone causing acoustic cancellation with the sound wave radiated from the front surface of the cone. In some enclosures, to minimize the radiation of reflected sound waves from the back surface of the cone, the enclosure is lined with sound-absorbent material. Another procedure for reducing the effects of the reflected back waves is to change the shape of the enclosure. In a triangular enclosure or an enclosure with angled walls, the sound wave from the back surface of the cone is reflected many times within the enclosure and is substantially diffused. In this way, the reflected waves that eventually radiate from the enclosure are not impluses but are much lower amplitude long pulses which are far less noticeable to the listener.
In another type of enclosure, the sound waves emitted from the back surface of the cone emerge from the enclosure in phase with the low frequency sound waves from the front surface of the cone. An enclosure of this type is the basic transmission-line enclosure which has a substantially rectangular outer structure. Sound wave radiation from the back surface of the cone flows down a conduit filled with low-density sound-absorbing material over a nonlinear path and emerges from a port or opening in the enclosure. Due to the time delay in traveling the conduit path and if the conduit is of sufficient length, the sound waves emerge in phase with the sound waves radiated from the front surface of the cone and do not cancel the rediation from the front of the cone.
A modification of the transmission-line enclosure utilizes vertical partitions within a rectangular enclosure to define a folded conduit with triangular cross-section. The triangular cross-section provides less audible coloration due to diffusing reflections and the sound waves are forced to travel down one triangular column of the conduit and up the next, and so on until they emerge from a port in the enclosure. In this modification of the transmission-line enclosure, small reflectors are placed in the corners of the triangular conduit to assist in directing the sound waves from the back surface of the cone through the conduit path.
Another type of speaker enclosure is the acoustical labyrinth. This enclosure operates on a principle similar to that of the transmission-line enclosure. The labyrinth enclosure comprises an absorbent walled conduit with one end tightly coupled to the back of the cone of the loudspeaker and the other end opening in front or at the bottom of the enclosure within which the conduit is folded. The length of the conduit is such that the radiation from the back of the cone emerges from the enclosure in phase with and additive to the front speaker radiation.
Another type of loudspeaker enclosure is the rectangular column which usually has a plurality of speakers positioned along the front or sides of the enclosure. In the column enclosure, the length of the column determines the level of performance achieved in the low frequency range. The lower the frequencies desired and the larger the speaker unit, the longer the column needs to be.
In each of the mentioned prior art enclosures, and for that matter, in every speaker enclosure, it is critical that the enclosure be constructed of sturdy material and firmly braced to prevent its panels from vibrating, rattling, or radiating sound waves into the listening room. When the enclosure comprises a long folded conduit or a plurality of partitions within a box-like outer enclosure, or when the side panels of a column speaker are quite long, it is difficult and expensive to construct the enclosure so that it is free from rattles and vibration. Naturally, the more complex the enclosure's structure the more expensive it is to manufacture. Also, since the quality of sound reproduction is frequently a function of the distance the sound waves must travel, and therefore the size of the enclosure, whehter it be the length of a folded conduit or the length of a column, the size and shape of the enclosure becomes a practical matter for consideration.
The folded column speaker enclosure of the present invention eliminates many of the problems presented by the above enclosures. It is an object of this invention to provide a sturdy, nonvibrating speaker enclosure that is relatively inexpnsive to construct and assemble because its shape is conducive to a two-piece hard plastic molded construction which can be assembled and positively bonded together with a minimum of bonding surfaces.
Another object of the present invention is to provide a compact enclosure having sound-reproduction capabilities of and a sound wave travel path of equivalent length to a column-type enclosure of greater volume.
Still another object of the present invention is to provide a visually aesthetic enclosure conducive to modular use in a total sound system, wherein the enclosure may be placed in a number of positions and configurations according to visual and audio desires of the listener.
A further object of the invention is to provide an enclosure that emits an additive dual resonant frequency from its port when a mid-range speaker is placed on the folded column substantially midway between the basic speaker and the port.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appending claims, and upon reference to the accompanying drawings.